This invention relates generally to gauging systems for measuring or sensing the level of liquid contained in a fuel tank, storage tank or the like.
In the past, a number of level indicators have been proposed and constructed. U.S. Pat. No. 2,398,375 illustrates one such system, showing a storage tank and an indicator device having conduits respectively connected to the top and bottom of the tank. The lower conduit is adapted to transfer the pressure existing at the bottom of the tank to a first diaphragm of the gauge. The upper conduit transmits the pressure at the top of the tank to a second diaphragm. Disposed between the diaphragms is a vertical chamber having a resistance wire running for substantially its full length, the chamber being filled with a column of mercury. Depending upon the relative pressure sensed by the two conduits, the level of mercury in the chamber changes, short circuiting either a greater or lesser length of the resistance wire. Indicator means connected to sense the resistance of this wire provides visual readings. By this arrangement, an indication of differential pressures between the top and bottom of the tank is obtainable, such an indication being independent of the absolute pressure existing outside of the tank. In addition, a plurality of contacts is disposed along the length of the chamber, such contacts being successively submerged as the height of the column of mercury increases. The contacts are connected to signal lights which in turn are energized by a battery having one lead in contact with the mercury column. The arrangement is such that as the level of mercury rises, it bridges additional contacts, thus causing the indicator lights to become successively illuminated. Each light corresponds to a particular pressure differential which can be converted into a corresponding level reading.
While this system was considered satisfactory at that time, several disadvantages became apparent later. Limitations on the permissible deflection of the diaphgrams impaired the accuracy of the device. In addition, the weight of the column of mercury acted against the weight of the liquid being gauged, and accordingly, liquids of different specific gravity resulted in erroneous readings. As a result, the unit had to be recalibrated each time a different liquid was incorporated in the tank. Also, the deflection of the diaphragms tended to be insensitive, and thus small changes in the level of fluid were not noticeable or measurable on the indicator.
Other systems employed a plurality of electrodes which were submerged in the fluid being gauged, together with bridge circuits of one sort or another adapted to measure the electronic conductivity or impedance of the fluid or capacitance between the electrodes, such readings being a direct function of the level of liquid in the tank. These systems were frequently dependent on the particular conductivity and/or dielectric constant of the fluid and thus required calibration each time it was changed. In addition, these systems lacked accuracy and were generally not useable with different liquids having wide ranges of conductive/dielectric characteristics.